1920] Allen: Plankton of the San Joaquin River 59 



acicularis being exceptional in that respect. It may also be the only 

 true planktont, all the others being adventitious. 



Pleurostauron parvulum Grun. 



station I Station II Station III Daily Hourly 



Average 34,372 261,925 149,609 310 28,521 



Identification probable. Probably very small percentage retained 

 by net. Almost entirely absent from Station I through July, August 

 and September. Pew during last half of year. Maximum in May 

 at Stations I and II and in March at Station III. Heavier represen- 

 tation in first half year at all stations. Hence it appears to be favored 

 by flood waters and lower temperatures. Larger numbers at Station 

 II and III indicate bad effects of sewage at Station I. Must be of 

 numerical importance. 



Stauroneis plioenicenteron Ehrbg. 



station I Station II Station III Daily Hourly 



Average 115 5,746 1,320 8,138 



Identification satisfactory. Losses through net probably heavy. 



Found at various times of year at all stations but in small numbers. 



Recorded only six times at Stations I and III. Maximum in May at 



Station II. Probably adventitious. 



Surirella spp. 



station I Station II Station III Daily Hourly 



Average 14,270 753,747 404,242 12,303. 305,828 



Genus identification certain. Probably at least four species in- 

 eluded under this heading. Losses through net heavy. Present 

 through year at all stations. Maximum in August at Stations II 

 and III. The representation at Station I was light and scattering 

 and though the maximum came in May it cannot be considered very 

 important, especially since large pulses came at that time at the 

 other stations. This planktont is evidently favored by temperatures 

 above 20° C. and retarded by sewage. Both in numbers and volume 

 it makes an important contribution to the plankton population. 



Synedra radians Kiitz. 



station I Station II Station III Daily Hourly 



Average 389,043 144,508 194,502 1,126,141 486,236 



Identification doubtful. Probably very small percentage retained 



by net. This is one of those very small forms so difficult to handle 



in a count. Its appearance on the record at mid-July is simply due 



to the fact that its continued presence seemed to demand some definite 



notice. The occurrence before that date must be ignored because of 



